The real option is a second small dieselengine, even outboard style, just to provide slow motion economically.
On my Cat, originally 27HP now 43 that gives me more than hull speed, or rather doesn't.
But a 5hp diesel would be really useful for plodding through a lull, just keeping steerage way for the auto-pilot, or charging the batteries during the night.
It's such a problem I'm considering a PETROL genny just to maintain lights and auto-p on the dark and shorter days of winter cruises.
Is there no viable diesel electric low power convertion unit besides a second hand car engine? There is a diesel quad bike 20hp, maybe skid-doo's already have the ideal diesel engine, light in weight, decent consumption, reliable starting. Life expectancy isn't relevant for sailing people?

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Ex Prout 31 Sailor, Now it's a 22ft Jaguar called 'Arfur' here in sunny Southampton, UK.
A few places left in Quayside Marina and Kemps Marina.

SurV
Check out www.asmomarine.com for the Thoosa electric motors. Your ship is 26 foot, right? You could reasonably throw out the diesel auxiliary and replace it with electric, IF (big letters) you do more daysailing than cruising, AND your Pearson can sail at 6 knots or better (which my 10 tonne 37 footer just can't). Bill

Max power is perhaps the most important number you need when looking at electric motors. Their power output is linear, so there is no real cruising rpm or torque curve. Once you hang a propeller off one there is. So if you are ruinnning a 72v eletric motor and want a 3 hp boost to your speed, you need 2237 watts or 31 amps at 72v. Even a 1hp boost will need 10 amps. Figure a group 31 battery has 100amp hours, so you could run a 3 hp boost for a little under 2 hours or a 1 hp boost for 5. Then do some math based on what you want to use to recharge to determine if it is worth it. You could design a really simple system with a solar panel pretty much directly connected to an electric motor. The boost, however, would likely be minimal. And electric motors don't help you point higher. In fact, motors of any kind move the relative wind closer to your bow, meaning you point lower to keep your sails filled.